When Gustav Mahler arrived in New York in the winter of 1907-8 to take up his post as principal conductor of the Metropolitan Opera, he came as the champion of Wagner’s Tristan und Isolde, Richard Strauss’s Also sprach Zarathustra and other immense masterworks, and as the composer of equally monumental symphonies.

It must have seemed somewhat incongruous, therefore, to those attending the New York Philharmonic Society concert of 10 November 1909, to see Mahler, by then the conductor of that orchestra as well, tuck his baton under his arm (as an eyewitness reported), sit down at a harpsichord and lead a performance of orchestral-suite music by Johann Sebastian Bach.

Mahler had assembled the score himself, taking music from Bach’s Orchestral Suites in B minor (BWV 1067) and D Major (BWV 1068), and producing a symphonylike arrangement of four movements. The fact that the new suite began in B minor (with the Overture from BWV 1067) and ended in D Major (with the Gavottes I and II from BWV 1068) might have violated Baroque convention, but it was fully in line with Mahler’s personal enthusiasm for ascending, minor-to-major key schemes, seen, for example, in the Resurrection Symphony of 1895 (which climbs from C minor to E flat Major). Like many other nineteenth and twentieth-century composers, Mahler did not hesitate to put his own stamp on Bach’s music when bringing it to performance.

Mahler’s admiration for Bach was intense and of long standing. According to his wife, Alma, the only scores he allowed in the summer house where he composed were the works of Bach. And in 1901 he confessed to Natalie Bauer-Lechner: ”It can hardly be expressed, what I learn more and more from Bach (admittedly as a child sitting at his feet), for my innate method of writing is Bach-like. If only I had time to immerse myself completely in this highest school!”

To this he added: ”I will dedicate my later days to him, when I am my own man.” In America, freed from the constraints of the Court Opera in Vienna and aware of his own fragile health, Mahler seems to have believed that the moment to express his passion for Bach publicly had arrived.

In making his suite arrangement, Mahler was following a path taken by many other musicians who were equally driven to update or improve Bach’s scores. Indeed, one can trace this path back to the composer’s own family: soon after Bach’s death, his eldest son, Wilhelm Friedemann, revamped many of the cantata scores for performances in Halle, adding, for instance, trumpet and timpani parts and a Latin text to two movements the cantata Ein feste Burg ist unser Gott (BWV 80). The brass parts were so attractive that they were printed with the work in the complete Bach Edition of the nineteenth century and are still included in many performances today.

The second-eldest son, Carl Philipp Emanuel, took great pains to preserve and champion his father’s music. Yet in 1786, when he paid homage to the Mass in B minor (BWV 232) by giving the premiere of the Credo section at a benefit concert in Hamburg, he did not balk at updating the work by adding an instrumental introduction of his own composition and by changing the instrumentation in a number of movements. Around the same time, Mozart arranged preludes and fugues from The Well-Tempered Clavier (BWV 846-93) for string ensemble, for performances at Baron Gottfried van Swieten’s famous ”Bach salons” in Vienna.

In 1802 the early Bach biographer Johann Nikolaus Forkel seems to have sounded the alarm for restraint, noting that the unaccompanied violin Sonatas and Partitas (BWV 1001-6), for example, were so perfect and complete in themselves that ”a second instrument was neither necessary nor possible.”

Mendelssohn and Schumann clearly thought otherwise. Mendelssohn wrote a piano accompaniment for the Chaconne of the Partita in D minor (BWV 1004) for a Leipzig Gewandhaus performance with the violinist Ferdinand David in 1841, and Schumann wrote piano accompaniments for all six of the unaccompanied violin works, and for the unaccompanied cello suites as well. Schumann remarked that Mendelssohn’s piano accompaniment of the D minor Chaconne sounded so fresh and convincing that ”the old, immortal cantor seemed to have a hand in the performance himself.”

And this is to say nothing of the electronic transformations of Wendy Carlos, the vocal renditions of the Swingle Singers and Bobby McFerrin, or the jazz interpretations of Jacques Loussier and Dave Brubeck. The list of Bach arrangements is lengthy indeed, and in the less dogmatic atmosphere of the post-”original forces” age, it appears to be getting longer. (Witness Ton Koopman’s recent reconstruction of the lost St. Mark Passion [BWV 247].)

What is it about Bach’s music that makes it such prime material for rearrangement? Why don’t we have a host of Mozart transcriptions or Brahms reorchestrations?

Part of the explanation can be found in Baroque musical practices, and in Bach’s compositional methods in particular. During the Baroque there was a strong tradition of musical borrowing, of using existing music as the basis for improvisation or new composition. A contemporary tells us that when Bach sat down at the keyboard, he would ”set his powers of imagination in motion” by playing something by another composer. Handel could scarcely pick up a pen without quoting someone else’s themes. Telemann liked to use the works of others, too.

In his youth, Bach reworked music by the day’s leading composers: Johann Adam Reincken (the Hamburg dean of German organists), Giovanni Legrenzi (the Venetian master of progressive trio sonatas) and Arcangelo Corelli (the Venetian codifier of the Baroque concerto). By fashioning fugues and keyboard transcriptions from their music, Bach acquainted himself with current styles and forms while finding his own artistic voice.

Later, as an established organ virtuoso at the Weimar court, Bach turned once again to keyboard arrangements, transcribing dozens of fashionable instrumental concertos by Vivaldi, Telemann, Benedetto Marcello and others. Here he appears to have competed with his cousin and colleague Johann Gottfried Walther to create keyboard transcriptions that captured the colors and contrasts of the Baroque instrumental ensemble.

When Bach became St. Thomas Cantor and town music director in Leipzig in 1723, he found himself under tremendous pressure to produce new works on a weekly basis, first for Lutheran church services and then for concerts of the university collegium musicum. During the initial years, he composed an extraordinary amount of music.

But he also began to recycle earlier pieces on a vast scale, arranging the music in brilliantly imaginative ways. New texts were inserted for old, outdated scorings were modernized, and instrumental concertos were transformed almost beyond recognition into cantata sinfonias, choruses and arias.

By the 1730’s, reworking old music had become a compositional way of life for Bach. The St. Mark Passion, the Christmas Oratorio (BWV 248) and the second book of The Well-Tempered Clavier (BWV 870-93) appear to have been produced largely through the recycling of existing material. The same is true of the harpsichord concertos, the four short Masses (BWV 233-6) and the Mass in B minor. Bach also arranged music by Palestrina, Caldara, Pergolesi and others, adding new touches and bringing the scores into line with his own style.

For many Baroque composers, revamping existing scores was a practical expediency. For Bach, it became a high art, an opportunity to enhance his own music and that of others, and carry it to a loftier level of perfection. Since absolute perfection could not be achieved by mortal man, the improvement of musical works was a never-ending process.

When Mozart and Brahms completed a piece, they closed the book and moved on to another project. For Bach, composition was a continuing affair, even with seemingly finished works. Hence, the Goldberg Variations (BWV 988) were augmented with a set of Fourteen Canons (BWV 1087), the Canonic Variations on Vom Himmel hoch da komm ich her (BWV 769) were given a new organizational scheme, and The Art of Fugue (BWV 1080) was expanded beyond its original design.

The transcendent values of Bach’s music – its melodic beauty, its contrapuntal strength, its rhythmic vitality, its harmonic profundity – speak across time, in a universal language, to a multitude of composers. But it is the embracing, inspiring open-endedness of his works that seems to move others to roll up their sleeves and try to carry Bach’s efforts farther.

It was in this spirit that Mahler appears to have approached his Bach orchestral-suite arrangement. He preserved the general text of Bach’s score, limiting his changes to the addition of dynamic markings, slurs and tempo gradations. He also shortened the value of detached notes here and there, to ensure uniform articulations.

In forte passages, he reinforced the solo flute with supplementary flutes and a clarinet, to produce a sufficient tutti in Carnegie Hall. (As it happens, Bach once did a similar thing: in the instrumental march of the cantata Vereinigte Zwietracht der wechselnden Saiten (BWV 207), a movement apparently used for a student processional in a large space, he asked that the parts be reinforced by as many as seven players.)

But Mahler also embellished Bach’s music with the addition of two written-out continuo parts, one for harpsichord, the other for organ. In the published score of 1910, he stated that the printed parts should be ”regarded as a sketch, which should bear . . . the characteristics of a free improvisation.” Mahler revived the suite arrangement several times with the Philharmonic, and Alma tells us that he altered the harpsichord accompaniment each time, ”according to his fancy.”

In the published version, we see that Mahler treats the harpsichord not as a steadily chordal instrument, in a Baroque way, but rather as a first-chair instrument that emerges here and there to add special splashes of orchestral color. This imparts a distinctly Mahleresque touch to the score.

Bach’s colleague Johann Mattheson seems to have had such accretions in mind when he advised composers that it was perfectly permissible to borrow someone else’s music, as long as it was returned with interest.